1. Field of the Invention
The present invention relates to a resin-coated sand and, more particularly, to a resin-coated sand suited for use in a laser sintering type rapid prototyping (RP) molding apparatus (laminate shaping apparatus).
This application claims priority based on Japanese Patent Application No. 2003-098142 filed on Apr. 1, 2003, the disclosure of which is incorporated by reference herein.
2. Background Art
In the production of castings, there has hitherto been widely used a so-called shell mold process of using a mold formed by charging a resin-coated sand, which is obtained by coating a refractory granular aggregate such as silica sand with a binder made of a phenolic resin, in a heated die.
This shell mold process is suited for mass production because a die is used in the production of a mold. However, in the case of multi-product production in small lots or the production of prototypes, the die must be fabricated each time, and thus the cost increases and many days are required to produce the die.
Under these circumstances, intense interest has been shown towards a RP system (rapid prototyping system) capable of directly forming a three-dimensional configuration input on a CAD (Computer Aided Design) system as a solid model (three-dimensional model) without machining using no die. As one of the RP systems, there is exemplified a laminate shaping method comprising repeating the step of applying a resin-coated sand on a work table to form a thin layer, and irradiating the thin layer with active energy ray such as a laser to form a sintered thin layer, and thus forming a mold. See, for example, Published Japanese Translation No. Hei 9-500845 of the PCT Application (WO 96/14203). In the laminate shaping method, a resin-coated sand for a shell mold has hitherto been diverted as it is, or there has been used a resin-coated sand wherein the amount of a phenolic resin to be coated is increased as compared with the case used for the shell mold process so as to cope with a poor strength.
In the laminate shaping method, however, the resulting mold may have a rough surface because of insufficient heat of laser during laser sintering (during laminate shaping). In particular, the undercut portion (portion with no support, the mold surface facing downward) has a very rough surface.
When using a resin-coated sand of the prior art, a phenolic resin increases the strength of the mold, but is converted into a gas when decomposed by heat of the molten metal during the production of a casing. Therefore, when the amount of the phenolic resin is increased so as to cope with a poor strength, the strength of the resulting laminate shaped article is improved. However, there arises a problem that the amount of a decomposed gas evolved from the phenolic resin (mold) increases and the gas penetrates into the molten metal, thus increasing gas defects of the resulting casting (product).
Therefore, use of such a RP system using the resin-coated sand is not increasing because the resulting casting is inferior in quality as compared with a shell mold process as a method of the prior art.
Under these circumstances, an object of the present invention is to provide a resin-coated sand which makes it possible to produce a mold with a less rough surface in a RP system and to produce a casing with fewer gas defects.